It is well known that photodynamic therapy or photochemotherapy is a therapeutic method which comprises administering a photosensitizer to a patient having pathological lesions, followed by irradiating a light of a wavelength active to excite the photosensitizer, to the pathological lesions where the administered photosensitizer has accumulated.
In a method of photodynamic therapy (abbreviated as PDT hereinafter) which is applied to the therapeutic treatment of cancer or tumor, there is administered such a photosensitizer which is capable of accumulating in the cancer or tumor tissue, particularly in the endothelial cell layers of the neovascular vessels present in the cancer or tumor tissue. Subsequently, the cancer tissue or the vascular walls containing the photosensitizer accumulated therein is irradiated with an exciting laser light active to excite photochemically the photosensitizer, so that the thus excited photosensitizer can further excite the oxygen molecules present in the surrounding regions around the photosensitizer. Through the excitation of the oxygen molecules, singlet oxygen (active oxygen) can be generated. Due to the cytotoxicity of the singlet oxygen, the cancer or tumor cells and the endothelial cells of the neovascular vessels containing the excited photosensitizer can be necrosed.
At an early development stage of PDT, hematoporphyrin derivatives such as photofrin, rose bengal and others were used as first-generation photosensitizers. The first-generation photosensitizers for use in PDT had such drawbacks that they have a poor selectivity to accumulate in the target tumor cells while they can readily accumulate in the normal cells, and that the first-generation photosensitizers can additionally involve a long-term phototoxicity in the skin of the patients [see, for example, Cancer Res., Vol. 38, pp. 2628–2635 (1978); and Cancer Res., Vol. 52, pp. 924–930 (1992)].
In recent years, the second-generation photosensitizers have been developed for PDT of cancer. Benzoporphyrin derivatives, mono-L-aspartyl chlorin e6 and others have been known as the second-generation photosensitizers [see, for example, a report of Kessel et al., entitled “Photodynamic therapy and bio-medical lasers”, pp. 526–530 (1992), Elsevier Science Publishers, Co., Amsterdam; and a report of Nelson et al., Cancer Res., Vol. 47, pp. 4681–4685 (1987)]. Mono-L-aspartyl chlorin e6 is one of the known fluorescent tetrapyrrol derivatives. The properties of mono-L-aspartyl chlorin e6 which is present in the cells in vivo have been examined in detail by W. G. Roberts [see a report of W. G. Roberts, entitled “Role of Neovasculature and Vascular Permeability on the Tumor Retention of Photodynamic Agents”, Cancer Res., Vol. 52, pp. 924–930 (1992)]. Based on the experiments of W. G. Roberts et al., with using chicken chorioallantoic membrane (abbreviated as CAM hereinafter) which is a tissue formed of the differentiation-type neovascular vessels of fertilized chicken egg, it was reported that the property of a photosensitizer to be selectively uptaken in the cancer cells and be accumulated therein can vary depending on the sort of the photosensitizer. In the above-mentioned reports, it is stated that mono-L-aspartyl chlorin e6 possesses a significantly higher selectivity to accumulate in the cancer cells, as compared with chlorin e6 and uro-porphyrin. Mono-L-aspartyl chlorin e6 or a salt thereof is able to absorb well a light of 664 nm-wavelength which is permeable deeply into animal tissue, so that the mono-L-aspartyl chlorin e6 compound can be photo-excited well in vivo after administration thereof. Additionally, it has been confirmed from some experiments with mice that mono-L-aspartyl chlorin e6 or a salt thereof can be metabolized and be cleared or eliminated from a living body of the host animal at a clearance speed of 10 times higher or more than that of the previously known hematoporphyrin derivatives, and that the concentration of mono-L-aspartyl chlorin e6 present in the plasma can decrease to a concentration of 1/100-fold of the initial concentration thereof in 10 hours after the first time of the intravenous administration of mono-L-aspartyl chlorin e6 [see the report of Kessel, et al., supra., entitled “Photodynamic therapy and Biomedical lasers”, pp. 526–530 (1992)]; and a report of C. J. Gomer and A. Ferrario et al., Cancer Res., Vol. 50, pp. 3985–3990 (1990)].
The above-mentioned experiments made by W. G. Roberts et al., with using chicken chorioallantoic membrane (CAM) have revealed that mono-L-aspartyl chlorin e6 tetra-sodium salt is able to be more selectively uptaken and accumulated at a higher concentration in the actively growing cells of the neovascular vessels in CAM, than such concentration at which the mono-L-aspartyl cholorin e6 salt can be uptaken and accumulated in the cells of the normal vascular vessels.
It has been indicated that mono-L-aspartyl chlorin e6 or a salt thereof has further characteristic properties that it is able to bind readily to the blood albumin and is hardly diffused in the normal tissue or normal vascular vessels having barriers, because mono-L-aspartyl chlorin e6 or a salt thereof has a low lipophilicity, and that the intracellular migration of the mono-L-aspartyl chlorin e6 compound will occur not through the physical diffusion but through the cellular endocytosis or cellular absorption.
Incidentally, Japanese Patent Publications Nos. 88902/1994 and 89000/1994 as well as their corresponding U.S. Pat. Nos. 4,675,338 and 4,693,885 describe, for example, mono-L-aspartyl chlorin e6 and mono-L-glutamyl chlorin e6 or salts thereof. And, these patents mentioned above describe also that the tetrapyrrol derivatives may be used as a photosensitizer for diagnosis and therapeutic treatment of tumor. In the aforementioned Japanese patents and USA patents, it is described that a fluorescent tetrapyrrol derivative which has been accumulated in the tumor tissue after the administration thereof can be photo-excited by irradiation of a potent light such as laser beam, so that the tetrapyrrol derivative so photo-excited can exert a necrotic action on the tumor cells.
There are known some experiments where the choroidal neovascular vessels of a monkey eye were destroyed by PDT with using rose bengal as a photosensitizer [Arch. Ophthalmol., Vol. 111, June/1993, pp. 855–860]. There are known additional experiments where the choroidal neovascular vessels of a monkey eye were occluded by PDT with using benzoporphyrin derivative mono-acid (Verteporfin as the photosensitizer) [Arch. Ophthalmol., Vol. 113, June/1995, pp. 810–818].
Furthermore, U.S. Pat. Nos. 5,705,518 and 5,770,619 of Richer et al., describe a PDT experiment where a photosensitizer, benzoporphyrin derivative mono-acid ring A (BPD-MA) is prepared as its liposome and is intravenously administered to a mouse having transplanted M-1 tumor, followed by irradiating an exciting laser beam to the mouse. Based on these experiments of Richter et al, there is proposed a method for destroying or impairing an area of neovascularization, which comprises transcutaneously irradiating said area with a laser light before an administered photosensitizer has permeated into dermal tissue or other normal tissues, so that the dermal phototoxicity can be avoided. In these patents of U.S. Pat. Nos. 5,705,518 and 5,770,619, Richter et al. refer to mono-L-aspartyl chlorin e6 as one example of the photosensitizer, and they additionally refer to that the method as proposed by Richer et al is possible to be applied to the destruction or impairment of the area of neovascularization as formed in the eye. However, there is nowhere disclosed any experimental Example which to show that any practical application of the method of Richer et al. was made to the field of ophthalmology.
The present inventors, namely Dr. Mori and Dr. Yoneya et al., previously proposed a method for photochemotherapeutically occluding neovascular vessels in the ocular fundus, and this proposed method was based on the results of their experiments wherein the normal vascular vessels in the ocular fundus of a normal pigmented rabbit were occluded, when the PDT with mono-L-aspartyl chlorin e6 tetra-sodium salt as a photosensitizer was applied to the normal vascular vessels (see U.S. Pat. No. 5,633,275). Furthermore, Dr. Mori and Dr. Yoneya et al., have carried out some experiments in which there are diagnosed such impairment of the retina and such occlusion of choroidal normal vascular vessel which had been involved when the PDT with mono-L-aspartyl chlorin e6 tetra-sodium salt as a photosensitizer was applied to the choroidal normal vascular vessels in the eye of Japanese monkey, with irradiating a laser light of 664 nm-wavelength at a fluence of 7.5 J/cm2 or less [Ophthalmology, Vol. 106, No. 7, pp. 1384–1391 (July, 1999)].
On the other hand, it is known that the neovascularization occurring in the ocular fundus tissue can severely damage the visual functions. For example, the choroidal neovascularization as involved by the age-related macular degeneration is a major cause for the intermediate blindness. Due to the occurrence of the choroidal neovascularization in the age-related macular degeneration, there are induced subretinal bleeding and subretinal exudation as well as retina detachment and fibrous proliferation, which can bring about a visual deterioration and occasionally blindness. Furthermore, it is known that when diabetes mellitus has progressed, a proliferative neovascularization is incurred in the retina, resulting in an onset of the proliferative diabetic retinitis and leading sometimes to blindness.
Hithertobefore, clinical therapeutic treatment of the neovascularization, which has occurred in the ocular fundus due to the age-related macular degeneration or the proliferative diabetic retinitis, has been done usually by making photo-coagulation of the neovascular vessels with irradiation of a laser light having a thermal action. It has been known that the photo-coagulation method made in the above prior art has a drawback that the thermal action of the laser light as employed can destroy even the surrounding normal tissues, whereas the neovascular vessels themselves can be occluded well by the thermal coagulation of them. However, if a selective occlusion of the neovascular vessels as formed in the ocular fundus could be made feasible by means of PDT with using a photosensitizer and a laser light irradiation, it can be expected that a satisfactory therapeutic method for selectively occluding the neovascular vessels in the ocular fundus would be developed and provided. In the past, therefore, a great number of research works have been carried out for further ophthalmological application of PDT.